By means of active immunization with a carcinogen-protein conjugate, we have been able to protect the Sprague-Dawley rat against chemically-induced neoplasia. In order to confirm the immunological mechanism of the protection, we intend to carry out parallel experiments using either passive or adoptive immunization, or both. Further investigation should also either confirm or rule out an indication from the published experiment, not statistically significant due to the size of the sample involved, that some non-specific protection was afforded animals immunized with a non-conjugated antigen. In other animal systems, several spontaneous tumor models exist for which a virus or other etiology is suspected; the role of a chemical etiology is not on this account rejected. Immunization of such animals using several different prosthetic groups on the antigen may, in view of our positive result in the rat, result in a statistical reduction of tumor production.